AC induction motors are widely used in automotive and industrial applications due, in large part, to their low cost, reliability, ruggedness, and simplicity. They typically consist of a stator and a rotor. The stator is a stationary member, and the rotor is a rotatable member positioned on a shaft within the stator. Coils are wound around both the stator and the rotor to form windings around each member. Applying an electric current to the stator windings produces a magnetic field that rotates at a frequency called the “synchronous frequency”. The rotating magnetic field induces currents in the rotor windings, which in turn, produce another magnetic field.
The two magnetic fields interact by trying to align themselves with each other. This interaction produces a torque, which urges the rotor to rotate. A maximum torque is achieved when the fields are furthest from alignment, and a zero torque is achieved when the fields are aligned (i.e., when the rotor rotates at the synchronous frequency). The difference between the rotational frequency of the rotor and the synchronous frequency is called the “slip frequency” and sometimes acts as a factor used in algorithms to control the speed of the motor.
Induction motors are usually controlled by manipulating the current running through the stator. One widely used control strategy is vector control, which relies on a mathematical representation of the current having a torque component (iq) and a rotor flux component (id). By applying a voltage to the current and modulating that voltage in response to feedback indicative of the torque and rotor flux components of the current, the speed and torque of the motor can be directly controlled.
Vector control strategy can effectively provide a quick response control of torque and speed, and increase the efficiency of the motor. However, because of its reliance on voltage modulation to control the current, vector control is relatively ineffective at high rotor speeds. At high rotor speeds, the current becomes less responsive to changes in the voltage, and the vector strategy loses control of current and torque. This can lead to overcurrent, overvoltage, undercurrent, oscillating torque, and generation of heat, all of which can damage the sensitive electronics of the controller and the motor.
U.S. Pat. No. 4,680,525 issued to Kobari et al. (Kobari) on Jul. 14, 1987, discloses a system and method that addresses the deficiencies of the vector control strategy at high rotor speeds. Kobari's system transitions from a vector control strategy to a slip control strategy when the rotor speed reaches a predetermined threshold. Unlike vector control, slip control does not modulate voltage to control the current in the stator. Instead, slip strategy controls the torque and speed of the motor by modulating the frequency of the current in response to the measured slip frequency of the motor. Once the rotor speed is below the threshold level, the system transitions back to a vector control strategy.
Although, utilizing a slip control strategy at high rotor speeds can compensate for some of the deficiencies of vector control, Kobari does not address the problems encountered during the transition between strategies. In particular, the same overcurrent, overvoltage, undercurrent, and heat generation problems experienced at high rotor speeds under vector control can often appear during the transition. In addition, an uncontrolled transition can lead to unpredictable torque and rotor speed. These problems can result in damage to the sensitive electronic equipment used in the motor control system and any apparatus being driven by the electric motor.
The present disclosure is directed towards overcoming one or more of the problems set forth above.